Single-Phase Symmetric-Bipolar-Type High-Frequency Isolated Buck-Boost AC-AC Converter with Continuous Input and Output Currents

A single-phase symmetric-bipolar-type high-frequency isolated buck-boost ac-ac (SBT-HFI-BBAC) converter is proposed. The converter is composed of two full-bridge inverter modules, input/output LC filters, and a high-frequency transformer (HFT) with series capacitors. It can provide identical noninverting and inverting buck-boost output voltages with symmetric-bipolar voltage gains of $ \pm D/({1 - D})$. The frequency of the output voltage can also be adjusted in discrete steps, similar to single-phase matrix converters (MCs). It supplies continuous input and output currents and works well with nonunity power factor loads. The magnetizing current of the HFT has no line frequency component due to the presence of the series blocking capacitors. Moreover, due to the absence of ac switches, the converter has a simple commutation process, unlike conventional ac-ac/MCs. Two switching strategies are proposed; Strategy-I with fewer high-frequency switches and Strategy-II with reduced switch currents, providing more flexibility in switch power loss optimization. The proposed SBT-HFI-BBAC converter is well suited for application as a dynamic voltage restorer, compensating a wide range of grid voltage sags and swells, without the need for a low-frequency voltage injection transformer. Its step-changed frequency operation can also find use in various applications, such as high-gain ac-dc converters, traction systems, etc. A comprehensive description of circuit operation is presented along with component design guidelines and appropriate comparisons. Experimental verification results are provided using a 400-W laboratory test circuit.